diff --git a/DESCRIPTION b/DESCRIPTION
index 3e2bfd5..a9db8a6 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -14,7 +14,7 @@ Authors@R: c( person("Eleonora", "Arnone", role = "aut",
 Maintainer: Aldo Clemente <aldo.clemente@polimi.it>
 Depends: 
     R (>= 3.5.0),
-    plotly, methods
+    plotly, Matrix, methods
 Description: Solving partial differential equations relying on the Finite Element Method.
 License: GPL (>= 2)
 URL: https://github.com/fdaPDE/femR, https://fdapde.github.io/femR/
diff --git a/vignettes/Introduction.Rmd b/vignettes/Introduction.Rmd
index fe14fe0..72f3ea5 100644
--- a/vignettes/Introduction.Rmd
+++ b/vignettes/Introduction.Rmd
@@ -77,7 +77,7 @@ data("unit_square", package="femR")
 mesh <- Mesh(unit_square)
 ## 2. Defining the solution of the PDE
 Vh <- FunctionSpace(mesh, fe_order=2) 
-f <- f <- Function(Vh)
+f <- Function(Vh)
 ## 3. Defining the differential operator
 L <- -laplace(f) 
 ## 4. Defining the forcing term and the Dirichlet boundary conditions as standard R functions